1. Field of the Invention
The invention relates to refrigeration. More particularly, the invention relates to vapour absorption refrigeration.
2. Description of Related Art
The invention provides a method of carrying out vapour absorption refrigeration, and a vapour absorption refrigeration installation. The invention provides also a method for operating an absorption stage of a vapour absorption refrigeration system and an absorption stage installation for a vapour absorption refrigeration installation.
One of two main heat transfer regimes is generally employed in refrigeration, namely vapour compression or vapour absorption. The present invention is concerned with the latter.
Both vapour compression and vapour absorption refrigeration cycles operate, generally speaking, on the basis of an inverse Rankine or inverse Carnot cycle. This involves, sequentially, (i) increasing the pressure of a gaseous refrigerant in a compression stage, (ii) condensing the refrigerant at constant pressure to liquefy the refrigerant, thereby reducing the heat capacity of the refrigerant, (iii) reducing the pressure either across a valve or through an expansion turbine to obtain partially evaporated refrigerant, and (iv) further evaporating the refrigerant by bringing it into a heat transfer relationship with a heat transfer medium, such as air, water or other fluids, that is at a higher temperature and is thus cooled through heat exchange with the refrigerant. Thereafter, the evaporated refrigerant passes back to the compression stage and the cycle is repeated. The heat transfer medium is cooled down or refrigerated.
A major difference between vapour compression and vapour absorption cycles is the manner in which the pressure of the refrigerant is increased. In a vapour compression refrigeration cycle, compression is achieved mechanically by means of a compressor, which constitutes the main work input requirement to the vapour compression refrigeration system. In a vapour absorption refrigeration cycle, however, the compressor is replaced by (i) an absorber, in which evaporated refrigerant is contacted with an absorbent, being a liquid, (ii) a pump which passes the refrigerant-enriched absorbent from the absorber to (iii) a generator or regenerator column, in which the refrigerant is fractionated out of the refrigerant-enriched absorbent and from which refrigerant-enriched vapour then exits as an overhead product at a high enough pressure to be condensed in a condenser.
From the brief exposition above, it follows that the work input to a vapour absorption refrigeration cycle or system comprises the work required to operate the circulating pump(s) and the work, or heat, required to evaporate and thus recover the refrigerant from the refrigerant-enriched absorbent. In a vapour absorption refrigeration system, it is therefore desirous to reduce the required work input to the system for providing a particular cooling effect, thereby to increase the coefficient of performance.
A number of factors impact on the coefficient of performance which is achievable by a vapour absorption refrigeration system. One such factor is the achievable regenerator overhead temperature, i.e. the temperature of the refrigerant leaving the regenerator. This temperature determines the operating pressure of the system and has a significant impact on the coefficient of performance, as an increased temperature requires that a greater fraction of chilling duty needs to be spent in cooling the refrigerant to the desired evaporation temperature.
Another factor that affects efficiency is the purity of the refrigerant that is recovered in the regenerator overhead product. This would impact particularly on the efficiency of the evaporator. More particularly, any absorbent contained in the overhead product would require some of the chilling duty to be expended on chilling the absorbent, rather than the refrigerant.
Yet a further limitation to efficient operation of vapour absorption refrigeration systems is the bottoms equilibrium temperature of the absorber, i.e. the temperature of the refrigerant-enriched absorbent. This temperature affects the pressure at which the generator must be operated, considering the vapour pressure of the refrigerant-enriched absorbent at the temperature at which it exits the absorber. More specifically, a higher absorber exit temperature would require a higher ratio of absorbent to refrigerant, thus increasing the required heat input to the regenerator.
What is also noteworthy is that absorption refrigeration systems typically operate by dissolving (i.e. absorbing) the refrigerant, for example ammonia, vapourised in the evaporator, at low pressure in solvent, e.g. water, and then separating the refrigerant, e.g. ammonia, from the water by distillation. The distillation pressure has to be high enough for the separated refrigerant to be capable of being condensed in the condenser at a temperature that is achievable with available cooling media. The consequence of this principle is that large quantities of solvent, e.g. water, are being heated up and cooled at temperatures where this heat input cannot be recovered and since, for example, water has a high latent heat of vapourisation the heat rejection tends to be high. With the compression refrigeration system, the only circulating fluid is refrigerant e.g. ammonia so that the heat load associated with circulating solvent e.g. water through the system, which is the case in absorption refrigeration systems, is avoided.
The present invention therefore seeks to reduce the amount of heat input required to carry out vapour absorption refrigeration whilst addressing the abovementioned process limitations, thereby to increase the coefficient of performance thereof.
In accordance with one aspect of the invention, there is provided a method of carrying out vapour absorption refrigeration, the method including
in a condensing stage, condensing a refrigerant in vapour form to obtain condensed or liquefied refrigerant;
passing this liquefied refrigerant into an expansion/evaporation stage;
in the expansion/evaporation stage, subjecting the refrigerant to heat transfer against a higher temperature medium such that heat is transferred from the higher temperature medium to the refrigerant, with the higher temperature medium being cooled and the refrigerant being heated such that at least some of the liquefied refrigerant is evaporated, thereby to obtain vapourised refrigerant;
passing the vapourised refrigerant from the evaporation stage into an absorption stage;
absorbing, in the absorption stage, some of the vapourised refrigerant into an absorbent at a first pressure thereby to obtain partially refrigerant-enriched absorbent;
contacting, in a compression absorption stage, the partially refrigerant-enriched absorbent with vapourised refrigerant under compression at a second pressure that is greater than the first pressure, thereby absorbing vapourised refrigerant into the partially refrigerant-enriched absorbent and obtaining refrigerant-enriched absorbent;
passing refrigerant-enriched absorbent from the compression absorption stage into a refrigerant regeneration stage;
in the regeneration stage, recovering refrigerant, in vapour form, from the refrigerant-enriched absorbent thereby to obtain refrigerant-depleted absorbent;
recycling recovered vapourised refrigerant from the regeneration stage to the condensing stage; and
recycling refrigerant-depleted absorbent from the regeneration stage to the absorption stage, with this refrigerant-depleted absorbent constituting the absorbent in the absorption stage.
By “higher temperature medium” there is meant a medium that is at a higher temperature than the refrigerant when the refrigerant enters the evaporator. The refrigerant can therefore be regarded as entering the expansion/evaporation stage at a temperature T1, being brought into a heat transfer relationship with the higher temperature medium at a temperature T2, with T1 being greater than T2.
It is also to be noted that, in the compression absorption stage, additional vapourised refrigerant is absorbed into the absorbent (being constituted by the partially refrigerant-enriched absorbent) to vapourised absorbent that had been absorbed into the absorbent in the absorption stage (in forming the partially refrigerant-enriched absorbent).
The absorbing of the vapourised refrigerant into the refrigerant-depleted absorbent in the absorption stage may include
contacting, in a pre-contacting stage, vapourised refrigerant with the refrigerant-depleted absorbent to obtain partially refrigerant-saturated absorbent; and
contacting, in a main contacting stage, vapourised refrigerant with the partially refrigerant-saturated absorbent, thereby to obtain the partially refrigerant-enriched absorbent.
More specifically, the pre-contacting stage may be a pre-saturation stage while the main contacting stage may be a pre-absorption stage. The process may then include
Passing vapourised refrigerant from the evaporation stage into the pre-absorption stage;
in the pre-absorption stage, absorbing vapourised refrigerant into an absorption liquid from the pre-saturation stage;
withdrawing unabsorbed vapourised refrigerant from the pre-absorption stage and passing it into the pre-saturation stage together with the refrigerant-depleted absorbent from the regeneration stage, with the refrigerant-depleted absorbent thus being pre-contacted and partially saturated with the vapourised refrigerant in the pre-saturation stage, thereby to obtain the partially refrigerant-saturated absorbent; and
withdrawing the partially refrigerant-saturated absorbent from the pre-saturation stage and passing it into the pre-absorption stage so that it constitutes the absorption liquid in the pre-absorption stage.
The method may include, in the pre-saturation stage, cooling the unabsorbed vapourised refrigerant and the refrigerant-depleted absorbent.
The pre-saturation stage and the pre-absorption stage may be operated at a pressure, being the first pressure, of between about 1.3 bar (abs) and about 4.5 bar (abs).
Subjecting the partially refrigerant-enriched absorbent and vapourised refrigerant to compression absorption in the compression absorption stage may include
withdrawing residual vapourised refrigerant from the pre-saturation stage;
withdrawing partially refrigerant-enriched absorbent from the pre-absorption stage;
passing the residual vapourised refrigerant from the pre-saturation stage and the partially refrigerant-enriched absorbent from the pre-absorption stage into the compression absorption stage in which the residual vapourised refrigerant is contacted with the partially refrigerant-enriched absorbent under increased pressure, thereby forming the refrigerant-enriched absorbent which is passed to the refrigerant regeneration stage.
Subjecting the partially refrigerant-enriched absorbent and residual vapourised refrigerant to compression absorption in the compression absorption stage may, in particular, be achieved by means of a compressor. Such a compressor may, in particular, comprise a liquid ring pump, with the partially refrigerant-enriched absorbent and residual vapourised refrigerant being passed to the suction side of the liquid ring pump and the refrigerant-enriched absorbent being withdrawn or discharged from the pressure side of the liquid ring pump. It is nonetheless expected that other types of compressors would also be suitable in carrying out compression absorption in the compression absorption stage, including but not limited to Archimedes screw-type compressors and vane-less impeller compressors. The invention is thus not limited to the employment of a liquid ring pump.
The residual vapourised refrigerant may be re-contacted with the partially refrigerant-enriched absorbent in the compression absorption stage at a pressure, being the second pressure, more particularly the discharge pressure of the pump, of between about 3.5 bar (abs) and about 10 bar (abs).
Contacting the refrigerant-depleted absorbent with the unabsorbed vapourised refrigerant in the pre-saturation stage may include subjecting the so contacted refrigerant-depleted absorbent and unabsorbed vapourised refrigerant to cooling heat transfer. Preferably, the contacting of the refrigerant-depleted absorbent with the unabsorbed vapourised refrigerant and the subjecting of the so contacted refrigerant-depleted absorbent and unabsorbed vapourised refrigerant to cooling heat transfer is effected simultaneously such that simultaneous mass transfer of unabsorbed vapourised refrigerant to refrigerant-depleted absorbent, and cooling heat transfer occurs.
In one embodiment of the invention, for a desired refrigeration temperature of between about 5° C. and about −30° C., the refrigerant may be condensed, in the condensing stage, to a condensed temperature of between 5° C. and 60° C. at a condensing pressure substantially equal to a regeneration stage operating pressure of between 5.5 bar (abs) and 25 bar (abs) by heat exchanging the recovered vapourised refrigerant from the regeneration stage against a coolant medium. The condensed refrigerant may then be supplied to the expansion/evaporation stage at a first pressure of between 6 bar (abs) and 26 bar (abs) and is then expanded or throttled in the expansion/evaporation stage to a second pressure of between 4.5 bar (abs) and 1.3 bar (abs), being substantially equal to the vapour pressure of the refrigerant for the desired refrigeration temperature. The condensed refrigerant may optionally be cooled to a temperature of between 34° C. and 0° C. by heat exchanging the condensed refrigerant against evaporated refrigerant from the expansion/evaporation stage prior to supplying the condensed refrigerant to the expansion/evaporation stage. The evaporated refrigerant may be supplied to the pre-absorption stage and the unabsorbed evaporated refrigerant to the pre-saturation stage at the second pressure and at a temperature of between 30° C. and 5° C. The refrigerant depleted absorbent from the regeneration stage may be fed to the pre-saturation stage at a temperature of between 10° C. and 60° C. and a pressure of between 2.0 bar (abs) and 5 bar (abs). The residual evaporated refrigerant may be subjected to compression absorption into the partially refrigerant enriched absorbent at a discharge pressure, being the second pressure, of between and 3.5 bar (abs) and 10.0 bar (abs). The refrigerant enriched absorbent may be fed to the regeneration stage at a temperature of between 5° C. and 70° C. Finally, the refrigerant may be evaporated out of and thus recovered from the absorbent, in the regeneration stage, at a temperature at the bottom of the regenerator of between 135° C. and 220° C. and at a pressure of between 5.5 bar (abs) and 25 bar (abs).
As has been alluded to above, the method may include, in the expansion/evaporation stage, and prior to subjecting the refrigerant to heat transfer with the higher temperature medium, expanding the condensed refrigerant across an expansion valve or mechanical turbine, thereby to obtain a mixture of liquefied refrigerant and vapourised refrigerant prior to subjecting the refrigerant to heat transfer against the higher temperature medium. The refrigerant mixture may then be subjected to heat transfer against another higher temperature medium, which may be refrigerant.
The refrigerant may, in particular, be ammonia. It is envisaged that the refrigerant may however, instead, be selected from a range of suitable refrigerants capable of being absorbed and thermally regenerated.
The absorbent may be a liquid having a higher boiling point than the refrigerant. The absorbent may, in particular, be water. It is envisaged that the refrigerant may however, instead, be selected from a range of suitable absorbents capable of absorbing refrigerants and being thermally regenerated.
As also indicated above, the refrigerant may be supplied to the condensing stage at a pressure that is substantially equal to the regeneration stage operating pressure.
The method may include supplying a cooling medium, typically air and/or cooling water, to the condensing stage to condense the refrigerant vapour. Preferably, the method includes supplying cooling medium to the condensing stage at a temperature and a flow rate sufficient to attain a condensing stage heat transfer duty which is, in turn, sufficient to condense substantially all the refrigerant passed to the condensing stage from the regeneration stage by cooling, at substantially constant pressure, the recovered refrigerant from a recovered refrigerant regeneration stage outlet temperature to a desired condensed temperature for the outlet pressure, which temperature may typically be about 35° C.
The method may further include accumulating condensed refrigerant in a condensed refrigerant accumulating stage. Typically, the accumulating stage may be at ambient temperature and at a pressure high enough to maintain the refrigerant in the liquid phase. In such an embodiment, the method may include supplying condensed refrigerant from the accumulating stage to the expansion/evaporation stage, more particularly to the expansion valve or expansion turbine, at a selected pressure, typically by generating a selected pressure head by means of a condensed refrigerant accumulating stage pump. The selected pressure may, preferably, be such that after expansion of the condensed refrigerant across the expansion stage an expanded pressure of the refrigerant will be substantially equal to the vapour pressure of the refrigerant for a desired refrigeration temperature, as quantified above.
In one embodiment of the invention, a condensed refrigerant stream is withdrawn from the accumulation stage and is refluxed to the regeneration stage.
The method may also include, prior to passing the condensed refrigerant to and expanding the condensed refrigerant across the expansion stage, thus upstream of the expansion stage, subjecting the condensed refrigerant to cooling heat transfer. Preferably, the condensed refrigerant is subjected to cooling heat transfer with the vapourised refrigerant which exits the evaporator.
In the evaporation stage, the refrigerant may be heat exchanged against the higher temperature medium which is desired to be cooled to approach or approximate to the evaporation stage operating temperature, i.e. the refrigerant saturation temperature at the evaporation stage operating pressure. Energy, in the form of heat, may therefore be exchanged, in the evaporation stage, between the higher temperature medium and the refrigerant, typically at the refrigerant saturation temperature thereby to vapourise substantially all the refrigerant and bring the evaporator substantially to the refrigerant saturation temperature, as quantified above.
The method may thus include operating the evaporation stage at a pressure substantially equal to the vapour pressure of the refrigerant at the desired refrigeration temperature. Thus, the vapourised refrigerant may be withdrawn from the evaporation stage at a substantially saturated condition with regards temperature, pressure and entropy.
Passing the vapourised refrigerant to the absorption stage, and more particularly to the pre-absorption stage, may therefore be carried out at a pressure substantially equal to, or typically slightly less than, the evaporator operating pressure. The method may therefore include operating the pre-absorption stage, and possibly also the pre-saturation stage, at a first absorption stage pressure which is substantially equal to, or slightly less than, the evaporation stage refrigerant operating pressure. It will be appreciated that when the vapourised refrigerant is heat exchanged against the condensed refrigerant upstream of the evaporation stage as hereinbefore described, the vapourised refrigerant will no longer be at the evaporator operating temperature.
Absorbing vapourised refrigerant into the absorption liquid in the pre-absorption stage may be carried out in a counter-current fashion, typically by means of a pre-absorption stage flash vessel into which the absorbing liquid from the pre-saturation stage is introduced operatively from the top of the flash vessel, whilst vapourised refrigerant is introduced operatively from the bottom of the flash vessel. Partially refrigerant-enriched absorbing liquid is thus obtained at the bottom of the flash vessel and unabsorbed vapourised refrigerant at the top of the flash vessel. The flash vessel may have suitable internal components for the disengagement of liquid and vapour as required by the process.
Absorbing vapourised refrigerant, typically unabsorbed vapourised refrigerant from the pre-absorption stage, may also be carried out, in part, in a flash vessel, also having suitable internal components for the disengagement of liquid and vapour as required by the process.
As alluded to above, the method may include removing heat from the absorption stage, more desirably from the pre-saturation stage, by means of cooling heat transfer. More particularly, the method may include, in a pre-saturation heat transfer operation, subjecting the unabsorbed vapourised refrigerant obtained from the pre-absorption stage and the refrigerant-depleted absorbent obtained from the regenerator stage to cooling heat transfer against a pre-saturation stage cooling medium in a pre-saturation stage heat exchanger, typically after the unabsorbed vapourised refrigerant and the refrigerant-depleted absorbent have been or whilst they are being so contacted or admixed. Such contacting or admixture can thus, in part, take place in the heat exchanger or can take place upstream therefrom. The method may include supplying the pre-saturation stage cooling medium to the pre-saturation heat transfer operation at a temperature and pressure to achieve a heat transfer duty which is sufficient to cool the unabsorbed vapourised refrigerant and the refrigerant-depleted absorbent, more particularly the mixture thereof, to a desired pre-saturator inlet temperature, typically about 35° C. The pre-saturator heat transfer operation may be conducted separately from the pre-saturator flash vessel, typically upstream therefrom and at or downstream of a mixing point at which the unabsorbed vapourised refrigerant and the refrigerant-depleted absorbent are contacted or admixed. As envisaged above, the mixing point may thus be at the pre-saturator heat transfer operation. Alternatively, the pre-saturator heat transfer operation forms part of the pre-saturator flash vessel, such that pre-saturation of the refrigerant-depleted absorbent from the regeneration stage with the unabsorbed vapourised refrigerant from the pre-absorption stage is carried out in the pre-saturation stage flash vessel with simultaneous cooling heat exchange and absorption (i.e. mass transfer). Holistically regarded, the pre-saturation stage may therefore involve simultaneous cooling (i.e. heat transfer) and absorption of refrigerant in refrigerant-depleted absorbent (i.e. mass transfer).
Contacting residual vapourised refrigerant from the pre-saturation stage with partially enriched absorbent from the pre-absorption stage in the compression absorption stage may include, as indicated above, the use of a liquid ring pump, with the compression absorption stage thus comprising a so called ‘liquid ring’ pump stage. The method may thus include centrifugally dispersing the partially enriched absorbent from the pre-absorption stage as a liquid ring in the liquid ring pump or liquid ring pump stage and introducing the residual vapourised refrigerant from the pre-saturation stage into the eye of the liquid ring, with the residual vapourised refrigerant thereby being absorbed into the partially enriched absorbent under increased pressure between vanes of an impeller of the liquid ring pump as these vanes intrude into the liquid ring. It will be appreciated that the pressure under which absorption is carried out in the compression absorption stage is higher than the operating pressure of the pre-absorption and pre-saturation stages and thus also higher than the operating pressure of the evaporation stage. This pressure has already been quantified above. As has also already been stated above, the liquid ring pump, although preferred, is not to be regarded as being exclusive in relation to the present invention as it is expected that other types of compressors could also, possibly, be employed.
The method may further include accumulating the refrigerant-enriched absorbent in a refrigerant-enriched absorbent accumulation stage upstream of the regeneration stage. Accumulation of the refrigerant-enriched absorbent in the refrigerant-enriched absorbent accumulation stage may be carried out at ambient temperature and at a pressure high enough to maintain the refrigerant-enriched absorbent in the liquid phase. In such an embodiment, the method may include, upstream of the refrigerant-enriched absorbent accumulation stage, subjecting the refrigerant-enriched absorbent to cooling heat transfer, typically by means of a cooling fluid such as water, to reduce the temperature of the refrigerant-enriched absorbent from an absorption stage exit temperature thereof to typically about 35° C.
When the method includes accumulating the refrigerant-enriched absorbent in the refrigerant-enriched absorbent accumulation stage, the method may also include withdrawing refrigerant-enriched absorbent from the accumulation stage and passing withdrawn refrigerant-enriched absorbent to the regeneration stage at a pressure substantially equal to the operating pressure of the regeneration stage, as has already been quantified above. Typically, in such a case, the refrigerant-enriched absorbent is passed to the regeneration stage by means of a pump which generates a pressure head sufficient to introduce the refrigerant-enriched absorbent into the regenerator substantially at the regenerator operating pressure.
Further, also typically when the method includes accumulating the refrigerant-enriched absorbent in the refrigerant-enriched absorbent accumulation stage, as a means of controlling the pressure in the accumulation stage, the method may further include introducing residual refrigerant vapour from the compression absorption stage into the vapourised refrigerant upstream of the absorption stage, more particularly upstream of the pre-absorption stage of the absorption stage. More particularly, the method may include withdrawing residual refrigerant vapour from a point upstream of the regeneration stage, typically from the refrigerant-enriched absorbent accumulation stage, and combining the withdrawn residual refrigerant vapour with the vapourised refrigerant from the evaporator stage.
The method may also include, typically when the refrigerant-enriched absorbent which exits the compression absorption stage has been subjected to cooling and accumulation, pre-heating at least some of the refrigerant-enriched absorbent prior to feeding the refrigerant-enriched absorbent to the regenerator stage.
Pre-heating the refrigerant-enriched absorbent may be carried out by subjecting at least some of the refrigerant-enriched absorbent to heat transfer, preferably by means of available heat generated by carrying out the method of the invention, i.e. against process streams which may have resulted by carrying out the method of the invention.
More particularly, the method may include, in a first refrigerant-enriched absorbent heat transfer stage, subjecting at least some downstream refrigerant-enriched absorbent to heat transfer with upstream refrigerant-enriched absorbent. Still more particularly, when the method includes accumulating the refrigerant-enriched absorbent in a refrigerant-enriched absorbent accumulation stage as hereinbefore described, the method may include, upstream of the regeneration stage but downstream of the accumulation stage, subjecting at least some accumulated refrigerant-enriched absorbent, which is withdrawn from the accumulation stage, to heat transfer against refrigerant-enriched absorbent which is being fed to the accumulation stage from the absorption stage.
Alternatively, or more preferably additionally, the method may include, in a second refrigerant-enriched absorbent heat transfer stage, subjecting at least some of the refrigerant-enriched absorbent withdrawn from the refrigerant-enriched absorbent accumulation stage to heat transfer with refrigerant-depleted absorbent obtained from the regeneration stage. More particularly, the method may include, when the refrigerant-enriched absorbent is accumulated in the accumulation stage as hereinbefore described, subjecting, upstream of the regeneration stage, at least some of the accumulated refrigerant-enriched absorbent which is withdrawn from the refrigerant-enriched absorbent accumulation stage and passed to the regeneration stage to heat transfer with the refrigerant-depleted absorbent from the regeneration stage.
In a preferred embodiment of the invention, the method includes splitting the refrigerant-enriched absorbent which is withdrawn from the refrigerant-enriched absorbent accumulation stage into first and second stream portions and, upstream of the regeneration stage, subjecting the first stream portion to heat transfer with upstream refrigerant-enriched absorbent and subjecting the second stream portion to heat transfer with refrigerant-depleted absorbent from the regeneration stage. The method may then include, after having been respectively subjected to heat transfer, feeding both the first and second refrigerant-enriched absorbent portions to the regeneration stage. Preferably, the method includes combining the first and second refrigerant-enriched absorbent stream portions upstream of the regeneration stage and feeding the combination of the first and second refrigerant-enriched absorbent stream portions to the regeneration stage. However, the first and second refrigerant-enriched absorbent stream portions can also be fed to the regeneration stage independently.
Recovering refrigerant, in vapour form, in the regeneration stage may include subjecting the refrigerant-enriched absorbent to fractional distillation. More particularly, the method may include, in the regeneration stage, heating the ref rigerant-enriched absorbent thereby to evaporate refrigerant out of the refrigerant-enriched absorbent and thereby to obtain the refrigerant-depleted absorbent, in liquid form, and recovered refrigerant, in vapour form.
Heating the refrigerant-enriched absorbent in the regeneration stage may include subjecting the refrigerant-enriched absorbent to heat transfer with a regeneration stage heat transfer medium. Thus, the method may include passing the regeneration stage heat transfer medium in a heat transfer relationship with the regeneration stage, and more particularly with the refrigerant-enriched absorbent in the regeneration stage. In one embodiment of the invention, the method may include, in the regeneration stage, passing at least two separate regeneration stage heat transfer medium streams in a heat transfer relationship with the regeneration stage, and more particularly with the refrigerant-enriched absorbent in the regeneration stage. The regeneration stage heat transfer medium may preferably be heated oil or steam. In a preferred embodiment of the invention, the method includes at least two regeneration heating stages configured as side re-boilers in addition to a main regenerator re-boiler located at the bottom of the column.
The method may also include, additionally to the one or more regeneration heating stages, subjecting the regeneration stage to heat exchange with refrigerant-depleted absorbent withdrawn from the regeneration stage.
The method may also include, in the regeneration stage, refluxing at least some refrigerant-depleted absorbent and subjecting it to heat exchange with a heating medium.
The heating medium may typically be an oil or steam, and may be at a temperature of between about 125° C. and about 275° C.
The cooling medium may typically be water or atmospheric air and may be at a temperature of between about 1° C. and about 50° C.
The refrigerant evaporation temperature, and thus the desired cooling temperature, may typically be between about −30° C. and about 5° C.
The applicant believes that the method of the invention, and particularly the approach suggested to absorbing vapourised refrigerant into refrigerant-depleted absorbent achieves a higher absorption efficiency, resulting in a greater fraction of the refrigerant-enriched absorbent being constituted by absorbed refrigerant. A purer regenerator stage overhead product is thus generated which, in turn, impacts positively on the achievable coefficient of performance by increasing the cooling effect that is achieved in the evaporator. Additionally, the applicant believes that the manner in which the method of the invention manages process heat utilisation allows for energy-efficient operation of a vapour absorption refrigeration system or installation that implements the method, particularly by reducing the required energy input to the system.
In accordance with another aspect of the invention, there is provided an absorption refrigeration installation which includes
a condenser in which a refrigerant in vapour form can be condensed, to obtain a condensed or liquefied refrigerant;
an expander/evaporator stage in which the refrigerant can be subjected to heat transfer with a higher temperature medium such that energy in the form of heat is transferred from the higher temperature medium to the refrigerant, with the higher temperature medium being cooled or refrigerated and the refrigerant being vapourised, with a condensed refrigerant transfer line leading from the condenser to the expander/evaporator stage;
an absorption stage in which the heated refrigerant can be absorbed, at a first pressure, into an absorbent to obtain a partially refrigerant-enriched absorbent, with a vapourised refrigerant transfer line leading from the evaporator stage into the absorption stage;
a compression absorption stage in which the partially refrigerant-enriched absorbent can be contacted with vapourised refrigerant at a second pressure that is greater than the first pressure, to obtain refrigerant enriched-absorbent.
a regenerator stage in which refrigerant can be recovered, in vapour form, from the refrigerant-enriched absorbent thereby to obtain a refrigerant-depleted absorbent, with a refrigerant-enriched absorbent transfer line leading into the regenerator stage;
a recovered vapourised refrigerant transfer line leading from the regenerator stage to the condenser and along which recovered refrigerant can be withdrawn from the regenerator stage and passed to the condenser; and
a refrigerant-depleted absorbent transfer line leading from the regenerator stage and along which refrigerant-depleted absorbent can be withdrawn from the regenerator stage, with this transfer line leading into the absorption stage so that the ref rigerant-depleted absorbent constitutes the absorbent in the absorption stage.
The absorption stage may include
a pre-contacting stage in which refrigerant-depleted absorbent can be pre-contacted with vapourised refrigerant to obtain a partially refrigerant-saturated absorbent; and
a main contacting stage in which the partially refrigerant-saturated absorbent can be contacted with vapourised refrigerant to obtain a partially refrigerant-enriched absorbent.
More specifically, the pre-contacting stage may comprise a pre-saturator, while the main contacting stage may comprise a pre-absorber stage. The absorption stage may then include
the pre-absorber to which vapourised refrigerant can be fed from the evaporator along the vapourised refrigerant transfer line and in which the vapourised refrigerant can be contacted with an absorption liquid from the pre-saturator and into which it can be absorbed to obtain the partially refrigerant-enriched absorbent, with an absorption liquid transfer line leading from the pre-saturator into the pre-absorber;
the pre-saturator to which refrigerant-depleted absorbent can be fed from the regenerator along the refrigerant-depleted absorbent transfer line and in which it can be contacted with unabsorbed vapourised refrigerant from the pre-absorption stage, thereby to pre-saturate the refrigerant-depleted absorbent, with an unabsorbed vapourised refrigerant transfer line leading from the pre-absorber to the pre-saturator.
A partially refrigerant-enriched absorbent and a residual vapourised refrigerant line may lead respectively from the pre-absorber and from the pre-saturator to the compression absorption stage. The compression absorption stage may comprise a compressor. In a preferred embodiment of the invention, the compressor may be a so called ‘liquid ring’ pump. For the liquid ring pump, therefore, the suction side supply may comprise (i) the partially refrigerant-enriched absorbent, supplied thereto from the pre-absorber along the partially refrigerant-enriched absorbent transfer line and (ii) the residual vapourised refrigerant, supplied thereto from the pre-saturator along the residual vapourised refrigerant transfer line. The compressor is, however, not limited to being a liquid ring pump and could, instead, comprise any other types of compressor suitable for achieving the desired compression absorption effect, e.g. an Archimedes screw-type compressor or a vane-less impeller compressor.
Thus, in use, residual vapourised refrigerant from the pre-saturator can be contacted with the partially refrigerant-enriched absorbent from the pre-absorber under increased pressure in the compression absorption stage, thereby to form the refrigerant-enriched absorbent which is passed to the refrigerant regeneration stage.
The expander/evaporator stage may include an evaporator in which the refrigerant can be subjected to heat transfer with a higher temperature medium such that energy in the form of heat is transferred from the higher temperature medium to the refrigerant.
The expander/evaporator stage may include an expansion device, typically in the form of an expansion valve or mechanical turbine across which the condensed refrigerant can be expanded to obtain a mixture of liquefied refrigerant and vapourised refrigerant. In such an embodiment, the condensed refrigerant transfer line may lead from the condenser to the expansion device and a refrigerant mixture transfer line may lead from the expansion device to the evaporator.
The refrigerant and absorbent may be as hereinbefore described.
The installation may include a cooling medium source from which cooling medium can be supplied to the condenser along a cooling medium line which leads from the cooling medium source to the condenser. The cooling medium may typically be water or air.
The installation may also include a condensed refrigerant accumulation vessel or tank in which condensed refrigerant can be accumulated, with a condensed refrigerant line leading from the condenser to the accumulation vessel and along which condensed refrigerant can be passed from the condenser to the accumulation vessel.
The installation may further include a condensed refrigerant pump for pumping condensed refrigerant, typically from the condensed refrigerant accumulation vessel, to the expander/evaporator stage, more particularly the expansion stage. The pump may be capable of generating a pressure head sufficient such that a pressure of the refrigerant after expansion across the expansion valve is substantially equal to the vapour pressure of the refrigerant at a desired evaporator stage operating temperature.
The installation may also include a condensed refrigerant heat exchanger upstream of the expansion valve with the condensed refrigerant transfer line leading through the refrigerant heat exchanger to the expansion valve. The vapourised refrigerant transfer line, leading to the absorber stage, may also pass through the condensed refrigerant heat exchanger such that heat can be exchanged between vapourised refrigerant from the evaporator and condensed refrigerant from the condenser in the condensed refrigerant heat exchanger.
The installation may include a higher temperature medium transport line which can carry higher temperature medium which is to be cooled through vapourisation of the refrigerant in the evaporator stage. The higher temperature medium transport line may therefore pass through the evaporator stage in a heat transfer relationship with the refrigerant mixture transfer line. The installation may thus be installed in a cooling relationship with a heat source which can transfer heat to a medium thereby to generate the higher temperature medium in the higher temperature medium transport line.
The pre-absorber may comprise a pre-absorber vessel/column. The vapourised refrigerant transfer line may then lead from the evaporator stage into the bottom of the pre-absorber column with the absorbent liquid transfer line leading from the pre-saturator into the top of the pre-absorber column. It will be appreciated that, in use, absorbent liquid is therefore contacted with vapourised refrigerant in a counter-current fashion in the pre-absorber and that partially refrigerant-enriched absorbent will therefore accumulate in the bottom of the pre-absorber column, with unabsorbed vapourised refrigerant passing to the top of the pre-absorber column. Thus, the partially refrigerant-enriched absorbent transfer line may lead from the bottom of the pre-absorber column and the unabsorbed vapourised refrigerant transfer line may lead from the top of the pre-absorber column.
The pre-saturator may include a pre-saturator vessel, preferably a pre-saturator flash vessel, in which the contacting of the unabsorbed vapourised refrigerant and the refrigerant-depleted absorbent is, at least in part, effected. The pre-saturator may also include a pre-saturator heat exchanger to which refrigerant-depleted absorbent and unabsorbed vapourised refrigerant are fed such that they are contacted with each other in the heat exchanger, the pre-saturator heat exchanger being capable of providing cooling heat transfer for absorption of the unabsorbed vapourised refrigerant into the refrigerant-depleted absorbent. In one embodiment of the invention, the unabsorbed vapourised refrigerant and refrigerant-depleted absorbent transfer lines may meet each other at a mixing point upstream of the pre-saturator flash vessel, preferably also upstream from or, more preferably, at the pre-saturator heat exchanger. Some absorption of the refrigerant into the refrigerant-depleted absorbent therefore takes place already upstream from the pre-saturator flash vessel, upstream from or in the heat exchanger. An inlet stream to the pre-saturator thus, in use, comprises a mixture of unabsorbed vapourised refrigerant and refrigerant-depleted absorbent, of which some of the unabsorbed vapourised refrigerant has already been absorbed into the refrigerant-depleted absorbent. Alternatively, the unabsorbed vapourised refrigerant and refrigerant-depleted absorbent transfer lines may lead separately into the pre-saturator vessel for the unabsorbed vapourised refrigerant and refrigerant-depleted absorbent to contact each other in the pre-saturator vessel, in which case the pre-saturator heat exchanger may be incorporated into the pre-saturator flash vessel.
The absorber stage may thus be provided with a cooling heat transfer arrangement, capable of removing heat generated in the absorber stage by means of cooling heat exchange. Typically, the absorber stage cooling heat transfer arrangement may include an absorber stage cooling medium line along which cooling medium can be supplied to the cooling heat transfer arrangement from the cooling medium source.
Preferably, as explained above, the absorber stage cooling heat transfer arrangement may be arranged in a heat transfer relationship with the pre-saturator. The absorber stage cooling heat transfer arrangement may then typically be arranged so as to exchange, in use, heat against a mixture of the unabsorbed vapourised refrigerant and the refrigerant-depleted absorbent.
The installation may further include a refrigerant-enriched absorbent accumulation stage, typically comprising a refrigerant-enriched absorbent accumulation vessel or tank. The refrigerant-enriched absorbent transfer line may thus lead to the refrigerant-enriched absorbent accumulation vessel, instead of to the regeneration stage. In such a case, the installation may include an accumulated refrigerant-enriched absorbent transfer line leading to the regenerator stage from the refrigerant-enriched absorbent accumulation vessel.
Further, typically when the installation includes a refrigerant-enriched absorbent accumulation vessel, the installation may also include a refrigerant-enriched absorbent pump which is capable of pumping refrigerant-enriched absorbent, typically from the refrigerant-enriched absorbent accumulation vessel, to the regenerator stage. The refrigerant-enriched absorbent pump may be capable of generating a pressure head sufficient such that the refrigerant-enriched absorbent can enter the regenerator stage at the regenerator stage operating pressure.
The installation may also include a refrigerant-enriched absorbent cooling heat exchanger in which the refrigerant-enriched absorbent which exits the absorption stage along the refrigerant-enriched absorbent transfer line can be heat exchanged against a cooling medium. The installation may thus also include a refrigerant-enriched absorbent heat exchanger cooling medium supply line leading from the cooling medium source to the refrigerant-enriched absorbent heat exchanger.
The installation may further include a refrigerant-enriched absorbent accumulation stage residual vapourised refrigerant transfer line which leads from the refrigerant-enriched absorbent accumulation vessel to the vapourised refrigerant transfer line that leads from the evaporator to the absorber stage, more particularly to the pre-absorber. In use, a mixture of residual vapourised refrigerant from the refrigerant-enriched absorbent accumulation stage and vapourised refrigerant from the evaporator may therefore be fed to the pre-absorber.
The installation may still further include at least one refrigerant-enriched absorbent pre-heating stage, typically comprising a number of refrigerant-enriched absorbent pre-heating heat exchangers for pre-heating refrigerant-enriched absorbent prior to the refrigerant-enriched absorbent entering the regenerator stage. More particularly, the installation may include a first refrigerant-enriched absorbent pre-heating heat exchanger to which the refrigerant-enriched absorbent transfer line or accumulated refrigerant-enriched absorbent transfer line, as the case may be, leads and in which the refrigerant-enriched absorbent transfer line or accumulated refrigerant-enriched absorbent transfer line is provided in a heat-transfer relationship with an upstream portion of the refrigerant-enriched absorbent transfer line.
The installation may alternatively, but more preferably additionally, also include a second refrigerant-enriched absorbent pre-heating heat exchanger to which the refrigerant-enriched absorbent transfer line or accumulated refrigerant-enriched absorbent transfer line, as the case may be, leads and in which the refrigerant-enriched absorbent transfer line or accumulated refrigerant-enriched absorbent transfer line is provided in a heat-transfer relationship with the refrigerant-depleted absorbent transfer line which leads from the regenerator stage to the absorber stage.
In a preferred embodiment of the invention, the refrigerant-enriched absorbent transfer line or accumulated refrigerant-enriched absorbent transfer line, as the case may be, is split between the first and second refrigerant-enriched absorbent pre-heating heat exchangers and is thereafter recombined before entering the regenerator. Alternatively, the split lines lead into the regenerator independently.
The regenerator may include a regenerator column, more particularly a fractionation column, with heating being provided at the bottom of the column to fractionate, in use, the refrigerant-enriched absorbent to recover refrigerant therefrom through fractional vapourisation. It will be appreciated that, in use, recovered vapourised refrigerant will accumulate in a top of the column and refrigerant-depleted absorbent will accumulate at the bottom of the column. Thus, the refrigerant-enriched absorbent transfer line or accumulated refrigerant-enriched absorbent transfer line, as the case may be, may enter the regenerator column substantially at a top portion thereof, with the recovered vapourised refrigerant line leading from the top of the regenerator column and the refrigerant-depleted absorbent line leading from the bottom of the reactor.
The installation may further include a regenerator stage heating heat transfer arrangement. The regenerator stage heating heat transfer arrangement may include at least one heating heat transfer medium line along which a heating heat transfer medium can be passed in a heat transfer relationship with the regenerator column to heat, in use, the regenerator column and thus the refrigerant-enriched absorbent to evaporate and recover refrigerant therefrom. Preferably, the installation includes, in the regenerator stage heating heat transfer arrangement, two heating heat transfer medium lines.
The installation may further include, also in the regenerator stage heating heat transfer arrangement, a refrigerant-depleted absorbent heat transfer line, which passes in a heat transfer relationship with the regenerator column.
In accordance with yet another aspect of the invention, there is provided a method of operating an absorption stage of a vapour absorption refrigeration system, the method including
contacting, in a pre-contacting stage, vapourised refrigerant originating from the evaporation stage with the refrigerant-depleted absorbent to obtain a partially refrigerant saturated absorbent; and
contacting, in a main contacting stage, vapourised refrigerant with the partially refrigerant saturated absorbent, thereby to obtain the partially refrigerant-enriched absorbent.
The pre-contacting stage may, in particular, be a pre-saturation stage and the main contacting stage a pre-absorption stage. The method may then include
passing vapourised refrigerant form an evaporation stage of the system into the pre-absorption stage;
in the pre-absorption stage, absorbing vapourised refrigerant into an absorption liquid from the pre-saturation stage;
withdrawing non-absorbed vapourised refrigerant from the pre-absorption stage and passing it into the pre-saturation stage together with the refrigerant-depleted absorbent from the regeneration stage, with the refrigerant-depleted absorbent thus being pre-contacted and partially saturated with the vapourised refrigerant in the pre-saturation stage, thereby to obtain the partially saturated absorbent; and
withdrawing the partially saturated absorbent from the pre-saturation stage and passing it into the pre-absorption stage so that it constitutes the absorption liquid in the pre-absorption stage.
The method may further include subjecting the partially refrigerant enriched absorbent and residual vaporized refrigerant to compression absorption in a compression absorption stage. The method may then include
withdrawing residual vapourised refrigerant from the pre-saturation stage;
withdrawing partially refrigerant-enriched absorbent from the pre-absorption stage;
passing the residual vapourised refrigerant from the pre-saturation stage and the absorbent from the pre-absorption stage into the compression absorption stage in which the residual vapourised refrigerant is contacted with the absorbent under pressure, thereby to form the refrigerant-enriched absorbent which is passed to the refrigerant regeneration stage.
The pre-saturation stage, pre-absorption stage and compression absorption stage may each be and may be carried out as is hereinbefore described.
In accordance with yet a further aspect of the invention, there is provided an absorption stage installation for a vapour absorption refrigeration installation, the absorption stage installation including
a pre-contacting stage in which refrigerant-depleted absorbent can be pre-contacted with vapourised refrigerant originating from the evaporator to obtain a partially refrigerant saturated absorbent; and
a main contacting stage in which the partially refrigerant saturated absorbent can be contacted with vapourised refrigerant to obtain a partially refrigerant-enriched absorbent.
More specifically, the pre-contacting stage may comprise a pre-saturator, while the main contacting stage may comprise a pre-absorber. The absorption stage installation may then include
the pre-absorber to which heated refrigerant can be fed from the evaporator along the heated refrigerant transfer line and in which the heated refrigerant can be contacted with an absorption liquid from the pre-saturator and into which it can be absorbed to obtain partially refrigerant enriched absorbent, with an absorption liquid transfer line leading from the pre-saturator into the pre-absorber;
the pre-saturator to which refrigerant-depleted absorbent can be fed from the regenerator along the refrigerant-depleted absorbent transfer line and in which it can be contacted with non-absorbed vapourised refrigerant from the pre-absorption stage, thereby to pre-saturate the refrigerant-depleted absorbent, with an unabsorbed vapourised refrigerant transfer line leading from the pre-absorber to the pre-saturator.
The absorption stage installation may further include a compression absorption stage, comprising a compression absorber, in which residual vapourised refrigerant from the pre-saturator can be contacted, under compression or pressure, with partially refrigerant-enriched absorbent from the pre-absorber.